Label-free biosensors based upon the detection of shifts in resonance wavelength, coupling angle, or the magnitude of optical resonances have become powerful, effective and commercially viable detection and analysis tools for pharmaceutical development, life science research, diagnostics, and environmental monitoring. See Cunningham, B. T. & Laing, L. L., Label-free detection of biomolecular interactions: Applications in proteomics and drug discovery. Expert Rev. Proteomics 3, 271-281 (2006); Fan, X. D. et al. Sensitive optical biosensors for unlabeled targets: A review. Analytica Chimica Acta 620, 8-26 (2008).
In evaluating the performance of biosensors, resolution is an increasingly important metric, as the ability to reliably measure small shifts in resonant wavelength (or angle) is required for detecting low concentration analytes, small molecule adsorption, and, ultimately, single molecules. In order to build high resolution label free biosensors that can detect small changes in adsorbed mass density, researchers have designed biosensor structures with passive optical resonators having small mode volume and cavity quality factor (Q-factor) values as large as 108, thereby reducing dramatically the shift in resonant wavelength of the sensor that can be reliably resolved. However, for high Q-factor passive resonator biosensors, sensitivity, as measured by the magnitude of wavelength shift, is compromised due to the high degree of confinement of the light inside the cavity. Fundamentally, sensitivity is determined by the strength of interaction between the evanescent electromagnetic field and the adsorbed biomaterial.
Recently, active sensors such as the DFB laser biosensor (DFBLB) have been demonstrated to produce intense and narrow bandwidth emission through the use of stimulated emission, while maintaining high sensitivity by the incorporation of a gain medium within the biosensor structure. See M. Lu et al., U.S. Patent application publication 2009/0179637; Lu, M., Choi, S., Wagner, C. J., Eden, J. G. & Cunningham, B. T. Label free biosensor incorporating a replica-molded, vertically emitting distributed feedback laser. Applied Physics Letters 92, 261502 (2008); and Ge, C., Lu, M., Jian, X., Tan, Y. & Cunningham, B. T., Large-area organic distributed feedback laser fabricated by nanoreplica molding and horizontal dipping. Opt. Express 18, 12980-12991 (2010).
External cavity diode lasers are described in some detail in the textbook of Ye, C. Tunable External Cavity Diode Lasers (World Scientific Publishing Co. Pte. Ltd., 2004). External cavity diode lasers are also described in the following publications: Saliba, S. D. & Scholten, R. E. Linewidths below 100 kHz with external cavity diode lasers. Appl. Opt. 48, 6961-6966 (2009); Fleming, M. & Mooradian, A. Spectral characteristics of external-cavity controlled semiconductor lasers. Quantum Electronics, IEEE Journal of 17, 44-59 (1981); Hawthorn, C. J., Weber, K. P. & Scholten, R. E. Littrow configuration tunable external cavity diode laser with fixed direction output beam. Review of Scientific Instruments 72, 4477-4479, doi:10.1063/1.1419217 (2001); Littman, M. G. & Metcalf, H. J. Spectrally narrow pulsed dye laser without beam expander. Appl. Opt. 17, 2224-2227 (1978).
In brief, external cavity lasers (“ECLs”) function as a single mode, narrow linewidth, and widely tunable semiconductor laser. A variety of configurations of external cavity lasers are known and described in the Tunable External Cavity Diode Lasers textbook. External cavity lasers are used in a wide variety of applications in coherent optical communication systems, ultra-high resolution spectroscopy, sensing, atomic clock timekeeping, and magnetometry. The most striking feature of the external cavity laser is its extremely narrow linewidth. The elongated resonator reduces the damping rate of intracavity light and the spontaneous recombination phase fluctuation, and therefore achieves low phase noise and narrow laser emission linewidth, with values typically below 1 MHz (0.0075 pm). Additionally, the high gain of a semiconductor laser allows for continuous wave operation, which permits simple detection, dynamic monitoring, and an inexpensive, small, robust electrical pump system. Typically, ECL systems utilize first-order diffraction from a grating to provide the optical feedback, as in typical Littrow and Littman-Metcalf configurations. Photonic crystal reflection filters have been demonstrated as efficient wavelength selective mirrors for ECL systems. See Chang, A. S. P. et al. Tunable External Cavity Laser With a Liquid-Crystal Subwavelength Resonant Grating Filter as Wavelength-Selective Mirror. Photonics Technology Letters, IEEE 19, 1099-1101 (2007).